1. Field of the Invention
This invention relates to augmenting mechanical excavation of earthen formations with high pressure fluid flow, and more particularly, but not by way of limitation, to drilling oil or gas wells with a mechanical bit from which a high pressure fluid jet is ejected.
2. Setting of the Invention
In one technique for mechanically excavating an earthen formation, such as in drilling an oil or gas well, a drill bit connected at the bottom of a drillstring is lowered and rotated against the rock of the formation to be drilled so that the rock is broken and a bore formed. A fluid is typically pumped through the drillstring and the bit to flush the broken pieces of rock from the bore.
One type of bit used in such mechanical excavation is referred to as a roller cone bit having a body to which conical cutting elements or members are mounted for rotation about respective axes of rotation. As the body of a roller cone bit is rotated with a drillstring to which it is attached, the cones roll along in contact with, and grind into, the formation to be excavated. This type of bit commonly includes fixed or stationary (relative to the bit body) openings or jets through which fluid pumped down the drillstring flows to flush the cuttings. A roller cone bit having two cones and one fixed cavitating jet nozzle is disclosed in U.S. Pat. No. 4,185,706 to Baker, III et al. Roller cone bits are also disclosed in U.S. Pat. No. 4,262,757 to Johnson, Jr. et al. and U.S. Pat. No. 4,518,048 to Varley (note also the background descriptions in these patents).
Other types of bits combining mechanical formation-engaging elements with fluid outlet structures are disclosed in U.S. Pat. No. 4,624,327 to Reichman, U.S. Pat. No. 3,393,756 to Mori, U.S. Pat. No. 3,548,960 to Hasiba and U.S. Pat. No. 4,534,427 to Wang et al. A substantial background description is given in the Wang et al. patent.
A conventional rock drilling technique typically combines, at least in the oil and gas industry, mechanical and hydraulic forces to do the excavating. Another type of excavating technique utilizing only hydraulic forces provides one or more high pressure jets ejected from a nozzle which is hydraulically or mechanically rotated about its own axis of rotation. See U.S. Pat. No. 4,031,971 to Miller, U.S. Pat. No. 4,175,626 to Tummel, U.S. Pat. No. 4,458,766 to Siegel, U.S. Pat. No. 4,119,160 to Summers et al, and U.S. Pat. No. 4,306,627 to Cheung et al.
Referring to the combined mechanical/hydraulic type of drilling, there is a particular technique within this category which provides a specific setting for the present invention. This technique, referred to as the FlowDril.TM. system, has been disclosed as comprising high-pressure intensifier pumps, a dual swivel, a dual conduit drillstring and a mechanical drill bit (such as a roller cone bit but not limited to roller cone bits) augmented with fixed high pressure nozzles. As disclosed, one stream of drilling fluid or mud is to be flowed, at about 40 gallons per minute and 20,000-30,000 pounds per square inch, through the dual swivel and a center tubing concentrically located inside normal drill pipe of the drillstring to the bit at the bottom of the hole where the fluid exits through the fixed nozzles as high-velocity jets. These jets slot the rock within narrow bands limited substantially to the widths of the jets. Another portion of the drilling fluid or mud is pumped through the annular space of the dual conduit drillstring between the center tubing and the normal drill pipe and out standard fixed nozzles in the bit. The exiting fluid streams are to mix at the bottom of the hole to form a drilling fluid with appropriate properties such as for well stabilization, cuttings removal and pressure control. See U.S. Pat. No. 4,614,327 to Reichman and U.S. Pat. No. 4,691,790 to Reichman et al.
The FlowDril.TM. system exemplifies that a high pressure liquid jet impacting on a permeable rock, but, not limited to permeable rocks, can remove rock at a significant rate. This occurs largely by the fluid increasing the pore pressure sufficiently to cause tensile failure of the cementing at the grain boundaries. It is also known that an impacting bit tooth creates fractures in the rock around the impact crater. This fractured rock can be removed by a high pressure jet in much the same way as the permeable rock is removed. By combining the hydraulic jets with the mechanical cutting, increased rates of penetration can be obtained whereby the excavation occurs more quickly. However, a significant problem with implementing this idea of high pressure liquid jetting with mechanical excavating in a single mechanical drill bit is that in order to obtain the high nozzle exit velocity that is required, the nozzle size must be very small and yet large enough not to plug with small debris in the drilling fluid. This dictates that only a few, such as from one to four, high pressure nozzles can be used. Because the jets are close to the bottom of the excavation, they do not have room to form widespread sprays and thus impact the bottom of the excavation in thin streams, thereby limiting the coverage of the bottom of the excavation to concentric circles formed by the narrow widths of the jets from the fixed nozzles. This shortcoming is exhibited in the disclosed FlowDril.TM. technique in that the narrow high pressure jet or jets of that technique cover a limited area of less than substantially all of the cross-sectional area to be excavated.
Therefore, there is the need for an excavation technique which provides a greater bottom hole coverage by a high-pressure fluid to augment better the mechanical forces imparted by a mechanical drill bit, such as a roller cone bit, through which the high-pressure fluid is ejected. It is contemplated that such an improved technique would produce greater rates of penetration. It is also contemplated that meeting such need could produce better cuttings removal and increased drill bit life (such as by the greater coverage of the bottom of the excavation creating more or better fluid-loosened material or by being able to obtain an acceptable rate of penetration at a lower load). Thus, it is contemplated that meeting such needs can reduce drilling costs.